network/discovery: Add to DHT only peers that support genesis-based protocol (#3833)

This PR adds to the DHT only the peers that support the genesis/fork/kad
protocol.
Before this PR, any peer that supported the legacy `/kad/[id]` protocol
was added to the DHT.

This is the first step in removing the support for the legacy kad
protocols.

While I have adjusted unit tests to validate the appropriate behavior,
this still needs proper testing in our stack.

Part of https://github.com/paritytech/polkadot-sdk/issues/504

.

cc @paritytech/networking

---------

Signed-off-by: Alexandru Vasile <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>